Techniques for punching a metal plate member in order to form a sprocket wheel having radial projections are known. However, punching is not suited for forming a bevel gear having teeth projecting in a direction that crosses radial directions, although a bevel gear is also a gear.
Grinding and cutting are known as techniques for cutting grooves to produce teeth in order to form a bevel gear. Forging and sheet metal stamping using a press device are also known as techniques for forming a bevel gear (refer to, e.g., Japanese Patent Application Laid-Open Nos. H11-188449 and 2001-205385).
The process of forming teeth of a bevel gear can be simplified by using the forging or sheet metal stamping technique to reduce the manufacturing cost if compared with the process of forming such teeth by using the grinding or cutting technique. Therefore, sheet metal stamping or forging is advantageous relative to grinding or cutting for forming bevel gears on a mass production basis from the viewpoint of manufacturing cost.
Members having a plurality of projections that look like teeth projecting in the same direction include, besides a bevel gear, a vibrating body of an ultrasonic motor having projections for amplifying vibrations. An ultrasonic motor has a vibrating body equipped with a piezoelectric element, which is a sort of electro-mechanical transducer, and is designed to generate a traveling wave on the surface of the vibrating body by supplying an alternating signal to the piezoelectric element and drive a moving body held in contact with the vibrating body by utilizing the traveling wave. Projections are formed on the surface of the vibrating body in order to boost the amplitude of the traveling wave generated on the surface of the vibrating body. Sheet metal stamping and forging using a press device are known as techniques for forming projections for amplifying the vibrations of the vibrating body (refer to, e.g., Japanese Patent Application Laid-Open No. H07-135785).
A press die having fins for forming grooves has to be provided in order to form a large number of teeth or projections projecting in the same direction on the surface of a metal object to be worked by forging or sheet metal stamping.
FIG. 21 is a schematic illustration of forming grooves on an object to be worked by sheet metal stamping. FIG. 21 illustrates a metal object 93 to be worked and fins 97a of a metal die.
Grooves are formed by applying a load on the metal object 93 to be worked by means of the fins 97a. In other words, the fins 97a are subjected to a heavy load. When narrow grooves are to be formed, thin fins 97a need to be used to form such narrow grooves. Then, the strength of the fins 97a is reduced so that the fins 97a may highly possibly be damaged when they are used for forging or sheet metal stamping. Therefore, the width of the grooves to be formed needs to have a certain large value in order to make the fins 97a show a certain degree of strength. Thus, the width of the projections of a bevel gear or a vibrating body to be formed in this way is subjected to a certain limitation.
Even if the metal die has a strength that is sufficient for bearing a single compression forming process, it is clear that the load to be applied in a single compression forming process is preferably small from the viewpoint of repeatedly using the metal die for compression forming. For this reason, the projections of a bevel gear or a vibrating body to be formed on a mass production basis are subjected to limitations in terms of their profile in order to reduce the load to be applied to the metal die.
Thus, the currently available methods of manufacturing a metal member having a plurality of projections have much room for improvement from the viewpoint of easily forming a plurality of projections projecting in the same direction with a desired width and reducing the load to be applied to the metal die in a compression forming process.